The present invention relates to the field of DC/AC conversion of electrical energy.
Inverters are supplied with a DC voltage and deliver as output an AC voltage by virtue of one or more transistor half-bridges. The output AC voltage is generally subjected to filtering.
Inverters of this type are, amongst other things, used for the electrical power supply of an X-ray tube.
An X-ray tube mounted, for example, in a medical radiology instrument, comprises a cathode and an anode which are both enclosed in an evacuated leaktight casing, so as to produce electrical insulation between these two electrodes. The cathode produces an electron beam which is received by the anode on a small surface constituting a focus from which the X-rays are emitted.
When a high supply voltage is applied using a generator to the terminals of the cathode and the anode, so that the cathode is at a negative potential -V and the anode is at a positive potential +V, with respect to the potential of the cathode, a so-called anodic current is set up in the circuit through the generator which produces the high supply voltage. The anodic current passes through the space between the cathode and the anode in the form of an electron beam which bombards the focus.
The anode is in the shape of a flat disc which is supported by a shaft, driven in rotation by a rotor of an electric motor, the stator of which is arranged outside the casing, with the aim of promoting the dissipation of the energy. The X-ray tube is arranged in an enclosure filled with an insulating refrigerant.
The characteristics of the X-rays which are emitted by the tube, in particular their hardness, depend on a number of parameters, including the value of the high voltage applied to the electrodes. This high voltage should be adjustable in order to obtain the desired characteristics, and should remain constant throughout the radiological exposure time, so as not to alter the operating characteristics of an X-ray receiver which receives the X-rays which have passed through the object which is undergoing examination.
X-rays tubes for medical diagnosis operate in pulses. It is therefore important for the time taken to establish the high voltage, as well as the time taken to return from this high voltage to a zero value, to be as short as possible.
A high-voltage generator for an X-ray tube generally comprises a supply circuit which delivers a DC voltage E starting with an AC voltage delivered by the mains. The voltage E is applied to the terminals of an inverter of the type which comprises at least one transistor half-bridge, each branch of the half-bridge comprising a switch S consisting of a transistor T and a freewheeling diode D mounted in antiparallel. The AC signal delivered by the inverter is applied, via a filter, to the primary of a step-up voltage transformer having a turns ratio k. The secondary of the step-up voltage transformer is connected to a rectifying and filtering circuit comprising at least one diode halfbridge and capacitors C.sub.f for filtering the voltage.
In known fashion, the inverter comprises a transistor pair connected in series to the output terminals of the supply circuit. A diode is connected between the collector and the emitter of each transistor T, so that its anode is connected to the emitter of the corresponding transistor. The bases of the transistors are connected to a control circuit which delivers switching signals for the transistors. In the case of a single half-bridge, the two output terminals of the inverter consist of the common point of the two branches of the half-bridge and of a point common to two capacitors of the half-bridge which are mounted in parallel and, in the case of two half-bridges, of each point common to the two transistors of a half-bridge.
The output filter of the inverter comprises, for example, a coil L.sub.r and a capacitor C.sub.r which are arranged in series, and a coil L.sub.p which is arranged in parallel with the capacitor C.sub.r. One of the terminals of the filter is connected to an output terminal of the inverter, and the other terminal is connected to a terminal of the primary circuit of the transformer. A filter with single resonance may also be used.
The rectifying circuit connected to the secondary of the step-up voltage transformer consists, for example, of a two-diode bridge, the point common to the two diodes being connected to one of the output terminals of the secondary of the transformer, two capacitors C.sub.f1 and C.sub.f2 being arranged in parallel with the diode bridge, the other terminal of the secondary of the transformer being connected to the point common to the two capacitors C.sub.f.
The control circuit essentially comprises a comparator, a circuit for measuring the current I.sub.1r. at the primary of the transformer, and a circuit for developing the switching signals for the transistors of the inverter. One of the two output terminals of the comparator is connected to the common point of two resistors of a voltage divider, to which the DC supply voltage V.sub.cf of the X-ray tube is applied, and the other is connected to a reference voltage source. The output terminal of the comparator delivers a signal whose amplitude is proportional to the difference between the two voltages applied to the input terminals, and it is connected to an input terminal of the circuit for developing the switching signals, so as to bring about a change in the frequency of the control signals for the transistors. The output terminal of the circuit for measuring the current in the primary of the transformer is connected to another input terminal of the circuit for developing the switching signals, with the aim of detecting and avoiding certain malfunctions of the inverter.
In conventional fashion, the control variable on which the control circuit acts is the time period T.sub.d until the transistors are turned on, starting from the instant when the current of the inverter reaches a zero value.
The presence of a filter with double resonance makes it possible to have the current of the inverter change as a monotonically increasing function of frequency, between the parallel resonant frequency and the series resonant frequency, the values of which depend on the values of the capacitor C.sub.r of the series coil L.sub.r and of the parallel coil L.sub.p of the filter. It therefore seems possible to control the power transmitted to the X-ray tube by the operating frequency of the inverter, and consequently the activation delay T.sub.d. A filter with single resonance also makes it possible to control the power by the activation delay T.sub.d.
However, known control circuits do not make it possible to set the DC supply voltage V.sub.cf of the X-ray tube to its desired value until too much time has elapsed, which results in time being wasted and by a dose of X-rays being received superfluously by the patient. This is because the efficiency of the X-rays for taking exposures is proportional to the voltage V.sub.cf raised to the fifth power. The dose of X-rays received before the desired voltage V.sub.cf is established, cannot be used for taking exposures.
These control circuits leave some degree of ripple in the voltage V.sub.cf after it has been established. This ripple is at a frequency of 100 or 300 Hz, depending on the supply type used: single-phase or three-phase. These ripples are even more problematic since the number of exposures taken per second may be as many as thirty and since it leads to an instability in the images. For a scanner, which is provided with a rotary source so as to obtain three-dimensional information, the image computation presupposes that the images are stable and consequently that the voltage is constant.